Download From populations to communities

From populations to
communities
Chapter 9 - new book
a fundamental question...
what determines a species’ abundance and
distribution
role of conditions and resources
migration
intra- and interspecific competition
predation and parasitism
combination of those
effects...
==> need to view the population in the context of
the whole community
why?
each exists within a whole web of interactions
each responds differently to abiotic conditions
•
Community matrix illustrating how each species may interact
with several others in competitive interactions
•
Plant species: 1, 2, 3
•
Grazes: 4, 5
•
Predators: 6, 7
•
Predator-prey interactions: 6 and 4; 5 and 2
Multiple determinants of the dynamics of
populations
Questions
•
•
•
Why are some species rare and
others common?
Why does a species occur at low
population densities in some
places and at high densities in
other places?
What factors cause fluctuations in
a species’ abundance?
Methodology: need to know…
•
•
•
physiochemical conditions, level of
resources available, organism’s life
cycle, and influence of competitors,
predators, parasites, etc
AND
how all these factors influence
abundance through birth, death,
dispersal and migration
Numbers…
data
•
•
•
What numbers cannot tell us
Some estimate of the numbers of
individuals in a population
But
A record of numbers alone can
hide vital information
•
•
•
3 human populations – with
identical number of individuals
One could be doomed to
extinction…one could grow
steadily..
What information is needed?
Need to estimate the
number of individuals +
different age, sex, and size
•
•
•
Many populations are stable
– but stability does not mean
nothing changes
(a) population dynamics of
annual sand-dune plant
during an 8 year study;
(b) irregular irruptions in
abundance of house mice in
agricultural habitat in
Australia. Abundance index
= # caught per 100 trapnights.
What to Emphasize?
Constancy…
fluctuations
•
•
We look for stabilizing forces within
populations to explain why the
populations do not exhibit
unfettered increase or a decline to
extinction
•
Example: density-dependent forces
•
We look for external factors,
weather or disturbance, to explain
the changes
Can the two sides be brought
together to form a consensus?
Is abundance determined or
regulated?
Determined
•
•
The precise abundance of
individuals will be determined by
the combined effects of all the
factors and all the processes that
affect a population – whether
dependent or independent of
density
Thus – must look at the
determination of abundance to
understand how it is that a
particular population shows a
particular abundance at a particular
time and not at another time
Regulated
•
•
Regulation is the tendency of a
population to decrease in size
when it is above a particular level
but to increase in size when it is
below that level
Can occur only as a result of one
or more density-dependent
processes that act on rates of birth
and/or death and/or movement
(a) Population regulation with: (i) density-independent birth and density-dependent death; (ii) densitydependent birth and density-independent death; and (iii) density-independent birth and death; (b)
population regulation with density-dependent birth and density-independent death. Death rates
determined by physical conditions with differ in three sites.
•
•
•
•
Some populations in
nature are almost
always recovering
from the last disaster
(a)
Others are usually
limited by an
abundant resource
(b)
Or by scarce
resource ©
Or by sudden
episodes of
colonization (d)
Key factor analysis
•
•
•
Calculate k-values for each
phase of the life cycle
Identified key phases (not
factors) in the life of a study
organism
k-values measure the
amount of mortality: the
higher the k-value, the
greater the mortality
•
k = killing power
For a key factor analysis: need a life
table
Static life table
Records numbers of survivors of different
ages – different numbers of
individuals in different age classes
Great care is required: can only be
treated and interpreted the same way
if patterns of birth and survival in the
population have remained much the
same since the birth of the oldest
individuals (which happens rarely)
Learn: age structure
Cohort life table
Records survivorship over time
Used more for annuals since nonoverlapping generations
List: age classes (or stages of organism’s
life); number of individuals surviving;
proportion of original cohort surviving
to start of age class; fecundity
schedules (number of seeds);
fecundity (mean number of youngest
age class produced per surviving
individual of each class; sum of
proportion and fecundity
Important questions
•
•
•
How much of the total ‘mortality’
tends to occur in each of the
phases?
What is the relative importance of
these phases as determinants of
year-to-year fluctuations in
mortality and hence of year-to-year
fluctuations in abundance?
•
•
Mortality during a key phase -- a
phase important in determining
population change – will vary with
total mortality
A phase with a k-value that varies
randomly with total k will have little
influence on changes in mortality,
and little changes in population
size
Regression coefficient: relationship
between phase mortality and total
mortality
Regression coefficient
Density-dependent
•
•
K-value should be highest
(mortality greatest) when density is
highest
Two phases for beetle population
•
Summer adults (key)
•
Older larve
•
What does all this mean?
Acorns, mice, ticks, deer and human
disease
•
•
•
•
Lyme disease caused by a
bacterium carried by ticks
Ticks live on mice and deer
– but can bite humans
# of mice, # of ticks, # of
deer – related to # of acorns
produced in oak forest
•
•
Big acorn crop  tick larvae 8 times higher
than poor acorn crop plus 40% more ticks
on each mouse
Mice and tick populations fall and rise with
availability of acorns
How?
•
•
Mice eat acorns
Abundance of mice when larval ticks
are active increases infected nymphs
Thus: risk of human exposure to Lyme
disease affected by mouse density in
the prior year and by acorn production
2 years previously
Movements…
•
Dispersal
•
•
•
Cannot ignore immigrants and
emigrants
•
Patches
•
•
Migration: can be a vital factor in
determining and/or regulating
abundance
Very important when populations are
fragmented and patchy
•
Habitable site or Dispersal distance
Need to identify habitable sites that
are not inhabited
Overall size of the pop can be
determined by accessibility of patchy
resource as by total amount of
resource
Metapopulations: dispersal +
patchiness
•
•
Metapopulation: comprises a
collection of subpopulations, each
one of which has a realistic chance
both of going extinct and of
appearing again through
recolonization
•
Less emphasis is given to the birth,
death, and movement processes
going on within a single
subpopulation
More emphasis is given to the
colonization (=birth) and extinction
(=death) of subpopulations within
the metapopulation as a whole
Temporal patterns in community
composition
•
•
Founder-controlled and dominance-controlled
communities
Community succession
•
Patch dynamics
•
•
Community organization is the focus
•
Combination of patchiness and dispersal  different dynamics than one
homogeneous patch
•
Disturbances – via gaps
2 different kinds of community organization
•
Founder controlled
•
•
All species are good colonists and essentially equal competitors
Dominance controlled
•
Some species are strongly superior competitively
Founder-controlled
•
•
•
•
Species are approximately equivalent
in their ability to invade gaps and can
hold the gaps against all comers
during their lifetime
Probability of competitive exclusion in
the community as a whole may be
much reduced where gaps are
appearing continually and randomly
Examples…
•
Tropical reef fish (in general)
•
•
•
Extremely rich in species
Great Barrier Reef (Australia) –
numbers range from 900 to 1500
species
Vacant living space is the crucial
limiting factor  unpredictability in
space and time when a resident dies
or is killed
Competitive lottery
•
Species richness maintained at high
level
Breed often; produce large clutches
of dispersive eggs; larvae are the
lottery tickets; first to a vacant space
wins
Competitive Exclusion
Principle
What it says
•
•
What it does not say
•
If two competing species coexist in
a stable environment, then they do
so as a result of niche
differentiation – i.e. differentiation
of their realized niches
If there is no differentiation
amongst their niches, or if it is
precluded by the habitat, then one
competing species will eliminate or
exclude the other
•
Does not say: whenever we see
coexisting species with different
niches we can jump to the
conclusion that this is the principle
in operation. Each species on
close inspection may have its own
unique niche
Need proof of interspecific
competition
•
Remove one species. Does the other
species increase its abundance or
survival?
Dominance-controlled
•
•
•
Some species competitively
superior
An initial colonizer of a patch
cannot necessarily maintain its
presence there
Disturbances that open up gaps
lead to reasonably predictable
sequence of species – because
different species have different
strategies for exploiting resources
 Community succession
•
•
Early species: good colonizers and
fast growers
Later species: tolerate lower
resource levels and outcompete
early species
•
 community succession
•
Early succession species  midsuccession  climax stage
Skip pages 302-306
Food webs
•
•
•
No predator-prey, parasite-host, or
grazer-plant pair exists in isolation
Each is part of a complex web of
interactions with OTHER
predators, parasites, food sources,
and competitors within its
community
We want to understand these food
webs
•
•
To understand the whole, we need
to have some understanding of the
component parts
Let’s focus on systems with at least
three trophic levels (plantherbivore-predator) and consider
not only direct but also indirect
effects that a species may have on
others on the same or other trophic
levels
Trophic
levels
Food webs
•
Deliberate removal of a species from a community can be a powerful
tool in unraveling the workings of a food web
•
Removal – might lead to an increase in the abundance of a competitor
•
Removal – might lead to an increase in the abundance of a prey
Or…
…Indirect effects..
•
•
•
Removal – might lead to a decrease in competitor
Removal – might lead to a decrease in the
abundance of a prey
Question is: what is more powerful: direct or
indirect effects?
Indirect effects…
•
Feral cats threaten native prey
(birds) with extinction on
islands
•
What to do?
•
Eliminate the cats?
•
•
What about the rats - which
have also colonized the island?
Removal of the cats would
relieve pressure on the rats –
and thus increase not
decrease threat to the birds
Trophic cascade
•
When a predator reduces the
abundance of its prey and this
cascades down to the trophic level
below – such that the prey’s own
resources increase in abudnance
•
One example: 2 yr experiment.
Predation by birds experimentally
manipulated in an intertidal
community on the NW coast of the
US to determine the consequences
for 3 limpet species (which the
birds eat) and their algal food
•
•
•
When birds are excluded ->
barnacles increase in abundance
at the expense of mussels, and 3
limpet species change in density
Removing predation by birds 
increased goose barnacle
abudnance + light-colored limpet
(L. digitalis)  decrease in mussel
area (due to competition)  might
have led to decrease L. pelta
(which eats mussels) but L.
strigatella is worse competitor so L.
pelta unchanged
Plus plant change: algal cover
decreased – because limpets eat
algae and barnacles compete with
algal colonization
Top-down or bottom-up
control of food webs?
Top-down
•
Predators control abundance of
herbivores and exert top-down
control
Bottom-up
•
Plants exert bottom-up control
In a trophic cascade, top-down and bottom-up
controls alternate as we move from 1 trophic
level to another
Top-down or bottom-up?
Why is the world green?
Or does the world taste bad?
•
•
•
Is the world green because topdown control predominates
green plant biomass accumulates
because predators keep herbivores
in check
•
•
Even if the world is green
(assuming it is), it does not
necessarily follow that the
herbivores are failing to capitalize
on this because they are limited,
top down, by their predators. Many
plants have evolved defenses
Herbivores may be competing and
their predators may be competing
World is bottom-up
Population and community
structure and food web
structure
•
Are there food web structures that are more stable than others?
•
What is stability?
•
•
•
A resilient vs a resistant community
•
Resilient: returns rapidly to former state
•
Resistant: little change
A fragile vs a robust stability
•
Fragile: remains unchanged but alters completely in large disturbance
•
Robust: remains roughly the same in the face of larger disturbances
Stability: in the face of disturbance
•
Disturbance – loss of one or more populations from a community
Keystone species
•
•
•
removal of this species would lead to significant
changes throughout the food web, producing a
community with a very different species
composition
one whose impact is disproportionately large
relative to its abundance
Can occur at any trophic level (not just predators)
•
•
Conventional wisdom: increased complexity =
increased stability
But mathematical models do not support this
argument
•
Conclusions differ depending on whether we
focus on individual populations within a
community or on aggregate properties of the
community (such as their biomass or
productivity)
Food webs…
1.
Number of species they contain
2.
Connectance of the web (fraction of all possible pairs of species that
interact directly with one another)
3.
Average interaction strength between pairs of species
•
•
Increases in # of species, increases in connectance, and increases in
average interaction – all tend to decrease the tendency of individual
populations within the community to return to their former state
following a disturbance (resilience)
Thus – community complexity leads to population instability
•
•
•
What is found in real
communities – and not in
models?
•
•
Keep in mind that:
•
•
The only communities we
can observe are those that
are stable enough to exist
Data on interaction strength
for whole communities are
unavailable – so assume
contant
Recent studies:
•
•
Connectance may decrease
with species number …or…
May be independent of
species number … or …
May even increase with
species number
Conclusion?
•
Stability argument does not
receive consistent support
from food web analyses
either
Summary of this chapter
•
•
there are multiple
determinants of the
dynamics of populations
Dispersal, patches and
metapopulation: movement
can be vital factor in
determining and/or
regulating number
•
•
There are temporal patterns
in community composition
No predator-prey, parasitehost or grazer-plant pair
exists in isolation